Our objective is to use parasite induced precocious host metamorphosis as a novel model system for probing developmental regulation. Parasitic wasps (Chelonus spp.) induce their hosts to precociously initiate the developmental program for metamorphosis through a mechanism of host regulation undescribed for either vertebrate or invertebrate parasites. In addition, the premature transition to the adult is subsequently blocked midway through. Further, the internal parasite need not be, and often is not, present when the effects of host regulation take place. The specific aims of the proposal are: A) Determine the basis for initiation and completion of metamorphosis in Trichoplusia ni. This aim will be accomplished by study of critical growth thresholds, changes in hormone titers and bioassays of tissue commitment during larval development. B) Delineate the mechanism by which Chelonus spp. induce precocious host metamorphosis, by examining changes in growth thresholds, hormone titers and tissue comitment in host larvae. C) Define the endocrine basis of cessation of metamorphosis during the premature prepupal stage. This effect of host regulation will be investigated by hormone titering, hormonal treatments parabiosis and implantation of active endocrine glands. D) Determine now these effects occurs even in the absence of the internal parasite and what is the causative agent(s) from the parasite. Various treatments to remove internal parasites or denature parasite secretory factors will show the source and nature of the agent(s).